Two-cycle engines generally have exhaust and intake ports in the side walls of a cylindrical combustion chamber (usually referred to simply as a cylinder). The intake ports or inlet valves feed an air and fuel mixture to the cylinder for combustion. The exhaust port opens to an exhaust passage system where the engine's combusted gases are released. Among its several functions, the reciprocation of the piston in the cylinder across each port cyclically seals and opens each port to effect the proper movement of gases through the engine. Since the ports' locations remain fixed within the cylinder, the exhaust port and intake ports are opened and closed at a fixed time and location with respect to the piston's movement during the engine's cycle.
The timing of the engine's cycle for opening and closing the exhaust port and intake ports directly affects the entire engine operation, including its horsepower, fuel efficiency, emissions toxicity, and even its ability to sustain continuous operation. In addition, optimal valve timing varies depending upon the engine speed and load. At high engine speeds, keeping the exhaust port open longer will improve engine performance. If such longer duration is permanently fabricated into the cylinder design, however, the engine will perform poorly at low speeds, where a comparatively shorter exhaust-port-open duration produces better performance.
Thus, unless one can vary the otherwise fixed timing of the exhaust port, the engine will only perform optimally at a certain rpm range. A number of attempts to remedy this problem have been made, including one disclosed in Applicant's U.S. patent application, Ser. No. 08/955,659, now issued as U.S. Pat. No. 5,873,334. Commonly these prior art engines provide some mechanism for covering the upper portion of the exhaust port under low speed operation, effectively lowering the top of the exhaust port in the cylinder to reduce the length of time that the port is open during the engine's cycle. Prior art engines also provide a mechanism to vary the height or axial extent of the exhaust port depending upon various parameters of the engine's operation.
Besides varying the exhaust port opening, prior art engines have also used various exhaust passage configurations to improve engine power and efficiency. Early exhaust systems featured a simple, straight exhaust pipe. These passages created a negative pressure wave to rid the cylinder of combusted gases and prepare the cylinder for a new charge. Later, it was found that the pressure created by the exhaust gas pulsation wave upon reflection from the open end of the exhaust pipe may be used to improve the engine's power and efficiency. A reflected pressure wave that arrives at the exhaust port with positive pressure may be used to urge an uncombusted air/fuel charge that has been discharged into the exhaust passage back through the open exhaust port into the interior of the cylinder. Alternatively, a reflected negative pressure wave may be used to pull spent gases out of the cylinder and prepare the cylinder for a new charge. Once an exhaust passage is fabricated, however, the effects of these pressure waves are optimal at only one particular engine speed. At other engine speeds, the geometry of the exhaust passage may result in the reflected wave returning too soon (before the port opens), or too late (sucking an uncombusted air/fuel charge out of the cylinder), thereby disrupting optimal engine operation.
Various solutions attempting to solve this problem have been proposed. For instance, U.S. Pat. No. 4,539,813, issued to Tomita et al., and U.S. Pat. No. 4,558,566, issued to Shirakura, each disclose engines employing a subsidiary chamber, or resonance chamber, in communication with the exhaust passage through a port in the exhaust passage. Such engines use a valve to open this resonance chamber port at low speed operation and close the valve at high speed operation. Thus, at low engine speeds that would otherwise create an undesired negative pressure wave at the exhaust port, the resonance chamber is opened to change such waves into positive pressure ones.
In many cases such prior art systems may respond inaccurately or relatively slowly to changing engine conditions, such as fast acceleration or deceleration. Instead, these resonance chamber valves often operate at fixed speeds.
Accordingly, it is desirable to more accurately vary the resonance chamber port opening. It would also be desirable to have an engine that employs both a variable exhaust valve system and a variable resonance chamber port system. Additionally, it would be desirable to have a single control module for adjusting both the exhaust port opening and the resonance chamber opening. It may be desirable to have the one control module respond to the same, sensed engine condition for adjusting the exhaust port opening and the resonance chamber opening. Also, it may desirable to use a single port valve for both the exhaust port and the resonance chamber port.